[ncmncm]

There are plenty of rocks from the late ("upper") Cretaceous, with a boundary containing elevated iridium. Suppose some microscopically thin boundary, microscopically lower, had a slightly different carbon isotope mixture. Who would notice, even looking? Would the two be even distinguishable, or hopelessly jumbled? Indeed, is anyone looking now? Would there be any reason to conclude that the difference was not also caused by the meteorite?